To conserve heated or cooled room air, it is common practice in exhaust pipe fume hoods to mix external air from outside a building with room air for sweeping fumes out of the building through an exhaust system. Such air mixing fume hoods are described in U.S. Pat. No. 3,340,788 and 3,747,504.
Both of the above patents describe fume hoods with vertically movable sashes to open and close a front opening of the fume hood. In FIGS. 9 and 10 of U.S. Pat. No. 3,747,504, structure is shown in which the vertically movable sash acts as a valve to direct the auxiliary air flow in front of the sash to help sweep fumes and room air into the fume hood when the vertical sash is open. When the vertical sash is closed, the major portion of external air flows behind the sash, directly into the fume hood, and can also help draw in some room air. This prevents the external air from dumping into the room when the sash is closed, putting additional burdens on the heating and cooling equipment of a building.
In certain fume hoods, it is desirable to have the sashes move horizontally. This is particularly true in a large walk-in fume hood in which the sashes, i.e. sliding doors, extend to the floor. U.S. Pat. No. 3,604,333 has attempted to provide a valving system in a horizontally movable sash in which a series of complex dampers are controlled by the movement of the sash. Because of the weight, suspension system, etc. of horizontally movable sashes, it is very difficult to provide a reliable valving system that does not put excessive drag on the horizontally sliding sashes.